For the crumb topping: Mix together granulated and brown sugar, salt, and cinnamon in a small bowl. Once they are all incorporated evenly, add in the butter, and continue to whisk until combined. Stir in the flour and mix with a rubber spatula until it looks like a thick dough. Set aside and let it cool for 10-15 minutes.
For the cake batter: Place the oven rack to the upper-middle position and preheat the oven to 325 F Degrees.
In the bowl of a standing mixer (or a hand mixer), fitted with the paddle attachment, mix the flour, baking soda, sugar, and salt on low speed. With the mixer running on low speed, add the butter pieces one by one until there are no visible butter chunks and it has a uniformed texture. (For a visual, please look at the pictures above).
Add the egg, egg yolk, yogurt, and vanilla and beat them until the batter is fluffy, resembling frosting, about 1 minute.
Cut a piece of parchment paper that is slightly less wide than an 8X8-baking pan. Spray the pan with vegetable oil spray. Place the parchment paper pushing it into the corners and up the sides. Let the excess to overhang the edges. This will help you get the cake out of the pan easily after it is baked. Spray the top of the parchment paper with vegetable oil.
Transfer the cake batter into the prepared baking pan and spread the batter into the pan evenly using a rubber spatula.
Using your fingers, break away the crumb mixture into small pebble size pieces. Evenly spread them on top of the cake batter starting from the outside corners and working your way into the center.
Bake it for 35 to 40 minutes, until the crumbs are golden and a toothpick inserted into the batter comes out clean.
Cool the cake (in the baking pan) for 30 minutes. Once cooled, remove it from the pan by lifting the parchment overhang. Dust it with confectioners’ sugar.
Cut it into 9 small squares and serve.